stormy rainy window effect
A polygon-rich approach
I gave it a go with a shower of particles. This may be a little bit too resource-intense for just a background feature though.
The setup
I created a simple &quot;room&quot; with

a double-layered window (two plane meshes);

a plane out in the sky, almost vertical, with a particle system;

two strong wind force fields that push the particles toward the window;

a simple icosphere as a dupliobject.



The building and the inner window layer are set as Collision objects with &quot;Kill particles&quot; activated.
The outer window is a Collision object with very high stickiness (no kill), and high damping/friction on particles.
The particle system has strong Normal velocity and low Gravity field weight, so that the rain drops don't fall off screen too quickly; &quot;Object&quot; &gt; &quot;Icosphere&quot; is selected as &quot;Render&quot; option, with &quot;Emitter&quot; un-checked.

(this preview is from an earlier version, with less particles and less friction on the surface)
The render
By giving the icospheres and window layers a Principled shader with very high transmission; adding some orange streetlamps and violet &quot;dusk&quot; sun (and a sad Suzanne in the storm); with some DOF and motion blur: this is a very rough result:

     (Only 7 frames)
Using nodes and a &quot;Geometry&quot; input, I made the raindrop's shader completely transparent (shadeless) when the &quot;y&quot; coordinate is less than y0 (the position of the window) to cut away the part of the sphere that &quot;permeates&quot; the first layer.
This answer's blend file



Even heavier for a close up shot (with metaballs)
If you want a close-up shot as in here, an additional Particle System with similar physics, but from a nearer plane (just near the camera) and with big meta-balls as DupliObjects, can help:

Result: